In order to print high resolution images on a light sensitive medium, various techniques have been developed to scan a laser beam over the light sensitive medium. A first technique is to deflect one or more laser beams over the light sensitive medium by means such as polygons or hologons. In this regard see, for example, U.S. Pat. No. 4,989,019 (R. P. Loce et al.), issued on Jan. 29, 1991, and U.S. Pat. No. 5,014,075 (Y. Okino), issued on May 7, 1991.
In certain cases, it is necessary to move the light beam across the light sensitive medium slowly because of low light sensitivity of the medium such as a dye transfer medium. In order to print at reasonable speeds in terms of prints per hour, multi-spot printers have been developed wherein, for example, a plurality of individually modulated light spots are separately moved across the medium.
U.S. Pat. No. 4,743,091 (D. Gelbart), issued on May 10, 1989, discloses a two-dimensional diode array for use in optical data storage. The array comprises rows and columns of laser diodes, each diode including a separate collimating lens. The diodes and the collimating lenses of the array are located on a spherical reference sphere and combined for imaging on an optical recording medium via a series of lenses. The optical recording medium is moving relative to the array image to provide scanning. Additionally, the rows of diodes of the array are offset relative to a previous row in a direction of the rows by an amount less than the spacing of the columns. An unrecognized problem with this two-dimensional diode array is that the size of spots produced at the optical recording medium vary due the different divergence angles of the beams from the various laser diodes of the array.
U.S. Pat. No. 4,681,394 (M. Noguchi), issued on Jul. 21, 1987, discloses a light beam scanning system representing another technique for scanning a plurality of laser beams over the light sensitive medium. More particularly, a plurality of laser beams emitted by a plurality of semiconductor lasers are combined into a single beam. The single laser beam formed from the combined laser beams is deflected to scan the surface to be scanned on the light sensitive medium. This technique, however, may not be suitable for some media such as a thermal dye media where certain power density limits have to be observed to prevent media decomposition. To meet such power density limits with a high density laser, the laser spot size has to be increased. This, however, causes a reduction in resolution of the image. A similar arrangement is disclosed in U.S. Pat. No. 4,924,321 (I. Miyagawa et al.), issued on May 8, 1990.
U.S. Pat. No. 4,907,017 (J. Azuma), issued on Mar. 6, 1990, discloses a laser optical apparatus representing a technique which uses first and second laser diodes where the laser beams therefrom are not combined. Instead, each of the first and second laser diodes transmits its laser beam through separate input optics and a separate set of scanning optics which share a deflector in the form of a polygonal mirror. The direction of the first and second laser beams are substantially parallel to each other.
U.S. Pat. No. 4,053,898 (K. Hirayama et al.), issued on Oct. 11, 1977, discloses a laser recording technique where one powerful laser transmits a light beam which is divided into a plurality of laser beams. More particularly, the plurality of laser beams are deflected and further divided into multiple beams. The multiple beams are arranged in a line at substantially regular intervals in a direction normal to a scanning direction of the multiple beams on a recording medium. Each beam is modulated and focused on the recording medium while the multiple beams are scanned over the recording medium. By forming multi-beams, high resolution is obtained. However, since only a fraction of the power of the single laser exists in each spot, the power in each spot is low.
U.S. Pat. No. 4,978,974 (M. Etzel), issued on Dec. 18, 1990, discloses an multi-spot imaging recorder wherein a linear array of laser diodes is mounted to project the output of the diodes onto a thermally activated photosensitive material movable along a predetermined path. The array of diodes is linearly aligned in a direction parallel to the path of movement of the photosensitive material. Additionally, the laser diodes are activated simultaneously as a group. Optical means are provided between the array of laser diodes and the photosensitive material to provide an imaging spot on the photosensitive material which is elongated in a direction perpendicular to the path of movement of the photosensitive material. More particularly, different laser diodes are disposed on a chip, and are separated by a relatively large distance to avoid cross talk between the diodes. A waveguide structure is used to provide an effective decreased distance between laser beams of the array of laser diodes.
A major problem with diode lasers is the fact that the divergence angles of the light can be different from one diode laser to another. These divergence variations cause spot-to-spot beam size variations at the print medium. It is desirable to provide a simple multiple diode laser printing system that includes a linear or two-dimensional laser array, avoids the use of optical fibers or waveguide structures, and corrects for divergence differences of the lasers.